A nonvolatile semiconductor memory device typified by a flash memory is used in various fields such as computers, communications, measurement devices, automatic control units and household appliances used around individuals, as an information recording medium that is large in capacity and small in size, so that there is great demand for the nonvolatile semiconductor memory device that is lower in price and larger in capacity. This is because, since data can be written electrically and data is not erased even when a power supply is cut, the nonvolatile semiconductor memory device can function as a data storage and a program storage in which initial setting to run portable devices such as memory cards and mobile phones is stored as nonvolatile data.
Meanwhile, in view of a great increase in application program and data itself in current circumstances, there is great demand for a system that can write software stored in the flash memory, fix bugs, upgrade the function, and the like. However, according to a conventional flash memory as the representative of the nonvolatile semiconductor memory devices, since it takes a long time to write data, and it is necessary to provide an extra storage region to buffer a file because data amount that can be written at one time is limited, the problem is that a processing procedure in writing the data becomes very complicated as a result.
In addition, flash memory is expected to face limit of miniaturization in principle, and thus research on new nonvolatile semiconductor memory devices that will replace flash memory has been widely carried out. Among them, a study of resistance change nonvolatile semiconductor memory device that utilizes the phenomenon that application of voltage to a metal oxide film causes resistance to change has been actively conducted recently, because the memory is more advantageous than flash memory in terms of limitation of miniaturization and because it is also capable of writing data at a high speed.
Although the study of the phenomenon that application of voltage to metal oxides such as nickel, iron, copper, titanium or the like changes resistance had been under way since 1960s (refer to Non-Patent Document 1), then, it was never put into practical use in actual devices. At the end of 1990s, it was proposed to apply to nonvolatile semiconductor memory device the fact that by giving voltage pulse for a short time to such oxides of manganese or copper having the Perovskite-type structure, deterioration of materials can be minimized and resistance can be irreversibly increased or decreased. Then, it was demonstrated that a memory array of memory unit devices (memory cells) in which variable resistive elements using these metal oxides were combined with a transistor or a diode could be really formed on a semiconductor chip. This was reported in IEDM (International Electron Device Meeting) in 2002 (refer to Non-Patent Document 2), which triggered wide research to be undertaken in the semiconductor industry. Later, a similar approach was also taken in the research on oxides of nickel or copper carried out in 1960s, and memory devices produced by being combined with a transistor or diode were also reported (refer to Non-Patent Documents 3 and 4).
All of these technologies are basically considered a same technology as they utilize resistance change in a metal oxide film to be induced by application of voltage pulse and use different resistance states as stored information in a nonvolatile semiconductor memory device (memory devices which constitute the nonvolatile semiconductor memory device).
Variable resistive elements (resistive elements made of metal oxides) whose resistance change is induced by application of voltage, as described above, exhibit various resistive characteristics or resistance change characteristics, depending on a material of a metal oxide (a metal oxide which changes its resistance by voltage application is referred to as a “variable resistor”, hereinafter), that of an electrode, form and size of a device, and operating condition. However, it is not known what causes the diversity in these characteristics. In other words, when researchers fabricated nonvolatile semiconductor memory devices, they simply made operating conditions that happened to exhibit the best characteristics as memory devices constituting a nonvolatile semiconductor memory device (referred to as a “nonvolatile semiconductor memory device”, hereinafter) operating conditions of those devices. Therefore, the overall picture of these characteristics has not been well understood, which still leaves us without any uniform design guideline.
Such condition without any uniform design guideline indicates that the above variable resistive element has not yet grown to be an industrially applicable technology in a true sense. In other words, in the empirically optimized technology as above, although the variable resistive element described above could be used as a single nonvolatile memory device or as a component in which the nonvolatile memory devices are integrated at a small scale, it cannot be applied to modern semiconductor devices that demand high quality assurance of large-scale integration of 1 million to 100 million units as with flash memory.
Specific instances the overall picture of which has not yet been understood, as described above, include bipolar (two polarities) switching characteristics and unipolar (unipolarity) switching characteristics. The switching characteristics of the both and applications thereof have already been reported in IEDM (refer to Non-Patent Document 2).
The bipolar switching implements switching between two resistance states by utilizing voltage pulses having two different polarities of positive and negative, having resistance of a variable resistive element transit from low resistance state to high resistance state with voltage pulse of any one of the polarities, and then having it transit from the high resistance state to the low resistance state with voltage pulse of the other polarity.
In contrast, the unipolar switching implements switching between two resistance states by utilizing voltage pulses having a same polarity and two different durations of long and short application (pulse width), having resistance of a variable resistive element transit from the low resistance state to the high resistance state with voltage pulse of one duration of application and then having it transit from the high resistance state to the low resistance state with voltage pulse of other duration of application.
Although so far there have been some reports on the both switching characteristics as described above, no report has done more than stating the characteristics in the operating conditions of any specific nonvolatile semiconductor memory device fabricated.
Each of the switching operations according to the above-mentioned two switching characteristics has advantages and disadvantages.
Since the bipolar switching can implement transit time of several 10 ns or shorter as resistance increases or decreases, a memory device utilizing this can write accumulated data at a very high rate. However, since both positive and negative voltage pulses are used, in order to operate only the selected memory cell while preventing a sneak path current, it is necessary to provide a selection transistor with respect to each memory cell (refer to FIG. 61).
FIG. 61 is a view showing a part of a memory cell array CA90 in which 1T1R type memory cells each including a variable resistive element and a selection transistor are arranged. A memory cell MC11 shown in FIG. 61 has a variable resistive element R11 and a selection transistor Tr11, and a predetermined voltage is applied to both ends of the variable resistive element R11 based on on-off control of the selection transistor Tr11. When it is assumed that a source line SL is the ground line, the voltage value applied to both ends of the variable resistive element R11 is determined by a voltage applied to a bit line BL1. In the case of the 1T1R type memory cell shown in FIG. 61, since an area per memory cell is increased as compared with the flash memory configured by 1T type memory cells, it is difficult to implement a memory device that is low in bit cost and superior to the flash memory.
In addition, although an attempt is made to reduce the area per memory cell configured by a variable resistive element showing the bipolar switching characteristics by combining with a two-terminal nonlinear element, in the nonlinear element in this case, a simple rectifying element cannot be used and very special characteristics are required. That is, as shown in FIG. 62A, when an applied voltage to both ends is changed, if the nonlinear element shows varistor characteristics in which a resistance value is lowered abruptly in a range where an absolute value is a certain voltage or more in either polarity, the above memory cell can be implemented in principle, but since an actual nonlinear element shows characteristics in which the resistance value is sequentially decreased as the absolute value of the applied voltage is increased as shown in FIG. 62B, it cannot show the ideal characteristics as shown in FIG. 62A. Accordingly, at this point in time, the memory cell using the nonlinear element having the characteristics shown in FIG. 62A cannot be implemented.
On the other hand, as the unipolar switching can implement switching operation with voltage pulses of a single polarity, circuit configuration can be simplified. In addition, as a combination of a diode and a variable resistive element (1D1R type) can be used, possible effect of a sneak path current from adjacent memory cells, which will be a problem when a memory cell array is configured as a cross point type, can be substantially reduced, thereby resulting in considerably improved electric characteristics in readout operation. FIG. 63 is a view showing a memory cell array CA91 configured by 1D1R type memory cells each having the variable resistive element and a diode serving as a two-terminal rectifying element. When this is compared with the 1T1R type memory cell shown in FIG. 61, the configuration of the memory cell can be simplified while preventing the sneak path current. Thus, the chip size can be reduced and the manufacturing cost can be reduced as compared with the configuration shown in FIG. 61, that is, the case of the bipolar switching.
However, as two long and short voltage pulses are needed in order to have the resistance state of the variable resistive element transit by the unipolar switching, and, in particular, the former one needs the pulse width of a few μs, writing thereof takes 100 times longer than that of the bipolar switching. In addition, since the memory cell current during writing ranges from about several hundreds μA to a few mA as with the case of the bipolar switching, to write each memory cell, the unipolar switching also requires about 100 times as high power consumption as the bipolar switching. Thus, it is severely inferior to the bipolar switching in terms of performance during writing. Since it is difficult to use means for erasing data in block and for programming a plurality of bits like the flash memory in view of chip power consumption, an operation speed of the single element exceeds the flash memory, but when the performance of the memory system is compared, performance difference in writing speed cannot be superior to the flash memory. Consequently, it is difficult to have a competitive edge over the flash memory.
On the one hand, in terms of stability of switching operations, there exist challenges in any switching characteristics. In order to start switching operations in a stable manner, voltage pulses having optimal voltage amplitude should be selected. However, the voltage amplitude must be determined through trial and error and according to characteristics of a variable resistive element. Thus, even in the case of the bipolar switching, stable switching operation can often be obtained by using voltage pulses to be applied that have different voltage amplitude as well as different polarities.    Non-patent Document 1: H. Pagnia, et. al, “Bistable Switching in Electroformed Metal-Insulator-Metal Devices”, Physica Status Solidi (a), 108, pp. 11-65, 1988    Non-patent Document 2: W. W. Zhuang, et. al, “Novell Colossal Magnetoresistive Thin Film Nonvolatile Resistance Random Access Memory (RRAM)”, IEDM Technical Digest, pp. 193-196, December 2002    Non-patent Document 3: I. G. Beak et al., “Highly Scalable Non-Volatile Resistive Memory Using Simple Binary Oxide Driven By Asymmetricunipolar Voltage Pulses”, IEDM, 2004    Non-patent Document 4: A. Chen et al., “Non-Volatile Resistive Switching For Advanced Memory Applications”, IEDM, 2005